Among the major environmental challenges facing society is the purification of water. Water is essential not only for the living body but also for industrial purposes. For these reasons there is a greater need to upgrade water purification technology.
Typical prior art systems to purify water fall into one of three categories. These are (1) ion exchange resin based systems, (2) ion exchange membrane based systems, and (3) EDI systems. These systems are discussed briefly below.
Ion Exchange Resin Based Systems:
The ion exchange resin adsorbs ionic species at their respective active sites. Once the active sites have been exhausted, the resin can be regenerated by washing the resin with acid or alkali to replace either H+ or OH− ions, respectively. This process is called regeneration of resin, and it is the main source for removal of cations and anions from various types of fluids. Demineralizing water is one of the most significant uses of this technology.
Ion Exchange Membrane Based System:
The ion exchange membranes are made of the same material as resin but work on a different principle. In the membrane based system, an electrical driving force is used to activate the movement of ions present in the water within a chamber of the anion and cation membrane. The ions are attracted towards the opposite electrical pole, then they meet the ion selective membrane, which allows selective movement of the ions. The cation membrane allows cations to move across the membrane and stops any anion from passing through. Similarly, an anion is allowed to pass through the anion membranes, but cations are prevented from further movement.
An electrodialysis process uses a voltage that is much below the point at which water splitting occurs in water. Electro dialysis has a limitation when it comes to treating water for high purity requirements because of high system resistance and its inability to remove ions like silica.
EDI System:
EDI is a technology that uses resin for its inherent ion adsorbing property along with the ion exchange membrane system of electrodialysis. EDI utilizes electrodialysis process along with resin as a conducting media introduced between the membranes. Normal EDI works on a very narrow band of feed water quality. The feed water quality required has to be equivalent to the product of reverse osmosis (“RO”), with hardness less than 1000 ppb. Some prior art teaches use of the RO prior to treatment (U.S. Pat. No. 6,379,518). Because of the inability of RO to provide this quality of feed water, softener also becomes a prerequisite.
In EDI, the resin has to adsorb the ions present in the water. The resin then releases these ions, giving them a definite mobility and direction towards the membrane. H+ and OH− ions have to be generated in the system. The regeneration of the resin caused by the H+ and OH− ions supplements the natural regeneration of resin due to the electrical driving force. Ions have to be transported across the membrane into the reject chamber. In water containing highly ionized species as hardness, along with weakly ionized species such as silica and carbonic acid, normal EDI is limited in its ability to remove all ionic species without a high probability of scaling in the system. The feed to EDI is generally limited, therefore, to feed water with hardness less than 1 ppm. Prior art EDI systems include those that make use of resin ion exchangers of various pore sizes (U.S. Pat. No. 6,471,867) or multiple types of resin in multiple deionization stacks.
In most feed water systems the impurities to be removed include strongly ionized cations, such as sodium and calcium, and strongly ionized anions, such as chloride and sulfate. Sodium and chloride ions have a cleaner removal compared to calcium, because the former ionized species are not chemically inclined to precipitate in EDI. Calcium and magnesium, on the other hand, are vulnerable to precipitation. Depending on the alkalinity of the system, calcium and magnesium convert to the hydroxide or carbonate form; the hydroxide and carbonate products then tend to precipitate.
Present EDI technology does not allow effective separation of mixed strongly and weakly ionic samples, because all ions are subjected to a uniform electrical driving force (U.S. Pat. No. 4,298,442, U.S. Pat. No. 6,391,178). For this reason the limiting condition for the presence of calcium or hardness in such systems is kept at the low limit of 1000 ppb.
Prior art EDI systems are unable to achieve the necessary hardness tolerance required to prevent scaling. Although such systems claim a maximum hardness tolerance of 1 ppm, the systems have been found vulnerable to scaling even at 1 ppm, limiting the usage of the process. Furthermore, some prior art systems designed to prevent scaling and reduce silica require the use of multiple deionizing apparatus and the addition of harsh chemicals (U.S. Pat. No. 6,398,965), the use of a nonstandard resin (U.S. Pat. No. 6,187,162), or the use of different types of resin in different stacks (U.S. Pat. No. 3,330,750; U.S. Pat. No. 3,149,061, U.S. Pat. No. 6,402,917). The present invention is directed toward overcoming one or more of the above-mentioned problems by offering a purification process that may be operated in one or more stacks, does not require the addition of harsh chemicals, and may utilize a uniform resin composition.